Glass inspection machines usually have a number of inspection stations at which a bottle carried through the machine will be inspected for defects. The machine may be an indexing, star wheel type machine. One or more of these inspection stations will include a stroke mechanism for displacing the inspection device from an up, retracted position, to a down, inspection, position. One such inspection device might be bottle specific gages mounted to a carriage. The carriage travels up and down on a linear guide system. The carriage is clamped to a timing belt that is supported by two pulleys. One of the two pulleys is attached to a gear head. The gear head is attached to a permanent magnet servo motor which is supplied by a suitable amplifier. The servo motor assembly contains a spring actuated, electromagnetically released, friction brake.
Machines will have an emergency stop button. If the E-Stop button is pressed or if the machine detects a dangerous condition, the machine stops the motion of all servo motors as fast as possible, pauses, drops the current that holds the motor brake in a released state, and releases the high voltage contractor (the motors are not moving when the brake is applied). In the E-Stopped state the star wheel is at a random position, and the stroke mechanisms are held at a random height. The star wheel may be moved by hand, the stroke mechanisms may not.
In the case of a power failure, the high voltage ceases to be available to the servo motors, the carriages continue in whatever direction they were headed. The current that holds the motor brake in a released state drops; the brake engages and stops the motor shaft from turning. This is the only condition that causes the brake to stop the rotation of the motor shaft. In the power failed state the star wheel is at a random position, and the stroke mechanisms are held at a random height. The star wheel may be moved by hand, the stroke mechanisms may not. The machine will remain in this state until power is restored and the start sequence is initiated.
The brake in the machine keeps the machine from damaging itself during a power failure and makes it easier to work on the machine during maintenance and set-up. If the stroke mechanisms did not have brakes, the mechanisms would move downward and hinder the technician's ability to move the star wheel during setup (E-Stop is recommended during set-up). The problem with the current design is that the brakes are not reliable. The brakes wear, create brake dust and fail. The failure causes the machine to shut down.